Process and apparatus for a nanovoided article

ABSTRACT

A process and apparatus for producing a nanovoided article, a nanovoided coating, and a low refractive index coating is described. The process includes providing a first solution of a polymerizable material in a solvent; at least partially polymerizing the polymerizable material to form a composition that includes an insoluble polymer matrix and a second solution, wherein the insoluble polymer matrix includes a plurality of nanovoids that are filled with the second solution; and removing a major portion of the solvent from the second solution. An apparatus for the process is also described, and includes a webline, a coating section, a partial polymerization section, and a solvent removal section.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/262,933, filed Oct. 5, 2011, now U.S. Pat. No. 8,808,811, whichclaims priority to U.S. Provisional Application No. 61/169,429, filedApr. 15, 2009, the disclosure of which is incorporated by reference intheir entirety herein.

BACKGROUND

Articles having a structure of nanometer sized pores or voids can beuseful for several applications based on optical, physical, ormechanical properties provided by their nanovoided composition. Forexample, a nanovoided article includes a polymeric solid network ormatrix that at least partially surrounds pores or voids. The pores orvoids are often filled with a gas such as air. The dimensions of thepores or voids in a nanovoided article can generally be described ashaving an average effective diameter which can range from about 1nanometer to about 1000 nanometers. The International Union of Pure andApplied Chemistry (IUPAC) have provided three size categories ofnanoporous materials: micropores with voids less than 2 nm, mesoporeswith voids between 2 nm and 50 nm, and macropores with voids greaterthan 50 nm. Each of the different size categories can provide uniqueproperties to a nanovoided article.

Several techniques have been used to create porous or voided articles,including for example polymerization-induced phase separation (PIPS),thermally-induced phase separation (TIPS), solvent-induced phasedseparation (SIPS), emulsion polymerization, and polymerization withfoaming/blowing agents. Often, the porous or voided article produced bythese methods requires a washing step to remove materials such assurfactants, oils, or chemical residues used to form the structure. Thewashing step can limit the size ranges and uniformity of the pores orvoids produced. These techniques are also limited in the types ofmaterials that can be used. There is a need for a rapid, reliabletechnique for producing nanovoided articles that does not require awashing step.

SUMMARY

In one aspect, the present disclosure provides a process for producing ananovoided article. The process includes providing a first solution thatincludes a polymerizable material in a solvent and at least partiallypolymerizing the polymerizable material to form a composition comprisingan insoluble polymer matrix and a second solution. The insoluble polymermatrix includes a plurality of nanovoids that are filled with the secondsolution. The process further includes removing a major portion of thesolvent from the second solution.

In another aspect, the present disclosure provides a process forproducing a nanovoided coating. The process includes coating a firstsolution on a substrate. The first solution includes a polymerizablematerial in a solvent. The process further includes at least partiallypolymerizing the polymerizable material to form an insoluble polymermatrix bicontinuous with a plurality of nanovoids and a second solution;the plurality of nanovoids being filled with the second solution. Theprocess further includes removing a major portion of the solvent fromthe second solution.

In another aspect, the present disclosure provides a process forproducing a low refractive index coating. The process includes coating adispersion on a substrate. The dispersion includes an ultraviolet (UV)radiation curable material, a photoinitiator, a solvent, and a pluralityof nanoparticles. The process further includes irradiating thedispersion with UV radiation to at least partially polymerize theradiation curable material, forming an insoluble polymer matrix bindingthe plurality of nanoparticles, and including a plurality of nanovoidsfilled with the dispersion depleted of the polymerizable material andthe nanoparticles. The process further includes removing a major portionof the solvent from the dispersion after at least partially polymerizingthe polymerizable material.

In another aspect, the present disclosure provides an apparatus forproducing a nanovoided coating. The apparatus includes a webline forconveying a substrate downweb from an unwind roll to a windup roll. Theapparatus further includes a coating section disposed proximate theunwind roll and capable of coating a first solution having apolymerizable material in a solvent onto the substrate. The apparatusfurther includes a polymerization section disposed downweb from thecoating section and capable of at least partially polymerizing thepolymerizable material to form a composition that includes an insolublepolymer matrix and a second solution. The insoluble polymer matrixincludes a plurality of nanovoids filled with the second solution. Theapparatus further includes a solvent removal section disposed downwebfrom the polymerization section, capable of removing a major portion ofthe solvent from the second solution.

BRIEF DESCRIPTION OF THE DRAWINGS

Throughout the specification reference is made to the appended drawings,where like reference numerals designate like elements, and wherein:

FIG. 1 is a schematic view of a process for forming a nanovoidedarticle;

FIG. 2 is a schematic view of a process for forming a nanovoidedarticle;

FIG. 3A is a schematic view of a process for forming a nanovoidedcoating;

FIG. 3B is a schematic view of a polymerization section of FIG. 3A;

FIG. 3C is schematic view of the polymerization section of FIG. 3B;

FIG. 4A is a scanning electron micrograph image of a coating;

FIG. 4B is a scanning electron micrograph image of a nanovoided coating;

FIG. 5 is a plot of refractive index versus UV LED power;

FIG. 6 is a plot of refractive index versus web speed;

FIG. 7 is a plot of refractive index versus UV LED power;

FIG. 8 is a plot of refractive index versus percent solids;

FIG. 9 is a plot of refractive index versus UV LED power;

FIG. 10 is a plot of refractive index versus photoinitiatorconcentration;

FIG. 11 is a plot of refractive index versus UV LED power; and

FIG. 12 is a plot of refractive index versus UV LED power.

The figures are not necessarily to scale. Like numbers used in thefigures refer to like components. However, it will be understood thatthe use of a number to refer to a component in a given figure is notintended to limit the component in another figure labeled with the samenumber.

DETAILED DESCRIPTION

A unique process and apparatus for creating nanovoided articles havingunique morphologies is described. The process is directed topolymerization of materials in a solution, while solvent is presentwithin the solution. The materials can be thermally polymerized, or theycan be polymerized using actinic radiation. A solution includingradiation-curable materials in a solvent can be particularly well suitedto the production of a nanovoided article. The solvent can be a mixtureof solvents, and particularly well suited solvent(s) are those that arenot reactive with the polymerizable materials. During polymerization,the solvent solubility of the formed polymer decreases and it separatesfrom the solution, resulting in an insoluble polymer matrix and a phaseseparated solvent rich network. The solvent is subsequently removed,leaving pores and voids which yield the nanovoided article. The solutioncan be coated on a substrate to provide a nanovoided coating on thesubstrate. In some embodiments, the substrate can be subsequentlyremoved leaving a nanovoided article.

Generally, as used herein, “pores” and “voids” refer to the polymer-freeregions within a nanovoided article that can be either partially, ortotally, surrounded by the polymer matrix. “Void” is a broader term thatrefers to any polymer-free region, no matter how small in volume, and isonly limited by the size of the nanovoided article. “Pore” is a subsetof “void,” and generally refers to a polymer-free region that extendssubstantially through the polymer matrix. A “pore” can be extensivethroughout the nanovoided article, and in some embodiments connects onesurface of the article to the other, as described elsewhere.

The effective diameter of any pore or void can be related to thediameter of a circle having the same cross-sectional area as the pore orvoid, and this effective diameter can be averaged over the dimensions ofthe article to provide an average effective diameter. Nanovoidedarticles can be “open-cell” structures, in which the pores or voids arein communication with the environment surrounding the article.Alternatively, nanovoided articles can be “closed-cell” structures inwhich the pores or voids are surrounded by the solid network or matrix,sealing them from the environment surrounding the article. In manycases, nanovoided articles include a combination of open-cell andclosed-cell structures.

The average effective diameters of pores and voids in a nanovoidedarticle can generally range in sizes less than about 1000 nm, less than100 nm, or even less than about 10 nm. In some applications,particularly applications including interactions with light, the averageeffective diameter of the pores and voids are comparable in size to thewavelength of light used. Several exemplary nanovoided articles and usesfor the nanovoided articles can be found, for example, in co-pendingU.S. application Ser. No. 13/264,325, entitled OPTICAL FILM; Ser. No.13/264,654, entitled BACKLIGHT AND DISPLAY SYSTEM INCORPORATING SAME;Ser. No. 13/264,254, entitled OPTICAL FILM FOR PREVENTING OPTICALCOUPLING; Ser. No. 13/264,281, entitled OPTICAL CONSTRUCTION AND DISPLAYSYSTEM INCORPORATING SAME; and Ser. No. 12/760,738, entitledRETROREFLECTING OPTICAL CONSTRUCTION, all filed on an even dateherewith. The use of the nanovoided article can be dependent on themechanical properties of the polymer matrix. In one particularembodiment, the polymer matrix modulus and strength are sufficient tomaintain a void space as the solvent is removed.

In some embodiments, the polymer matrix modulus and strength areinsufficient to maintain a void space after the solvent is removed,resulting in a “collapsed” coating without nanovoids. In one suchembodiment, the homogeneous composition includes a polymer gel. Apolymer gel is a polymer network that is expanded throughout its wholevolume by a fluid (in this case the solvent), but is not self-supportingafter removal of the solvent. Such a collapsed coating can provideimprovements in the production of a homogeneous coating with reducedcoating defects, as described for example in U.S. application Ser. No.13/258,029, entitled PROCESS AND APPARATUS FOR COATING WITH REDUCEDDEFECTS, filed on an even date herewith.

The present process permits the ability to control the distribution ofthe pores throughout the article. For example, the pores and voids inthe nanovoided article can be uniformly dispersed throughout thearticle, non-uniformly dispersed such as in a gradient, or they can varyin size, shape, and distribution throughout the article. In oneparticular embodiment, at least a portion of the pores and voids arecontinuous throughout the article, i.e., there is a continuous butpotentially tortuous path connecting each pore and void to the surfacesof the article. The continuous path (often resulting from a bicontinuousphase) permits ready removal of solvent from the article, rather thanthe solvent becoming trapped in a closed-cell structure duringpolymerization of the polymer matrix.

In one particular embodiment, the polymerization apparatus uses recentlydeveloped ultraviolet light emitting diode (UV LED) systems. UV LEDsystems can be small in size and radiate very little infrared radiation,which can result in less heating of the coating. These characteristicsmake it safe and practical to expose UV-curable compositions in anenvironment where a coating solvent is present. UV LED systems can beconfigured to operate at several desired peak wavelengths, such as 365nm, 385 nm, 395 nm, 405 nm, and the like. Other radiation sources may beused, such as, for example, UV lasers, UV lamps, visible lamps,flashlamps, and the like; other high-energy particle devices can beused, including, for example, electron-beam (EB) sources and the like.

The polymerization can occur rapidly, and the polymerization apparatuscan be placed between a coating station and conventional solvent removalsystem. The polymerization apparatus can also be placed withinconventional drying equipment or between a series of conventional dryingequipment, as long as there is still a significant portion of thesolvent present within the coated film at the onset of cure.

Processing parameters can affect the resulting nanovoided article,including, for example, web speed, coating thickness, UV LED spectrumand peak wavelength, intensity, dose, temperature, and composition ofthe coating at the onset of polymerization. Other processing parametersthat can affect the resulting nanovoided article include composition ofthe coating during polymerization, and environmental control, including,for example, gas phase composition, gas flow fields, and gas flow rates.Gas phase composition can include both solvent composition andconcentration, and oxygen concentration particularly near thepolymerization region. Control of the coated film environment fromcoating application through the polymerization process is desired, andcan be accomplished with temperature-controlled enclosures with bothsupply and removal of conditioned gas. In some cases, simultaneouscuring (polymerization) and drying can occur. The drying technique mayalso affect the thin film morphology and uniformity.

The polymer matrix should have sufficient modulus and mechanicalintegrity to maintain a void space after removal of the solvent. In someembodiments, the polymer matrix is a crosslinked matrix, such as athree-dimensional polymeric matrix, that resists deformation during andafter solvent removal. Particulate fillers (e.g., particles such asnanoparticles) can be added to the polymer matrix to affect theformation and strength of the nanovoided article. In some cases, theaddition of nanoparticles can increase the effective modulus of thepolymerized material, increase or decrease the pore/void averageeffective diameter and distribution throughout the article, decrease theconversion of the polymerizable material at the gel point, increase theviscosity of the solution before and during cure, or a combination ofthese and other effects.

In some embodiments, the process for creating the nanovoided coatingsgenerally includes 1) supplying the solution to a coating device; 2)applying the coating solution to a substrate by one of many coatingtechniques; 3) transporting the coated substrate to a polymerizationapparatus (the environment can be controlled to deliver the thin filmcoating at the desired composition); 4) at least partially polymerizingwhile solvent is present within the coating (the polymerization can beperformed in ambient conditions or in controlled environments); 5)optionally supplying conditioned gas upstream, downstream, or within thepolymerization apparatus to control the polymerization environment; 6)transporting the polymerized coating to drying equipment (drying cannaturally occur during this transport step unless equipment is in placeto prevent it); 7) drying the polymerized coating; and 8) optionallypost-processing the dried polymerized coating, for example, byadditional thermal, visible, UV, or EB curing.

FIG. 1 shows a schematic view of a process 100 for forming a nanovoidedarticle 170 according to one aspect of the disclosure. A first solution110 that includes a polymerizable material 130 in a solvent 120 isprovided. The polymerizable material 130 of the first solution 110 is atleast partially polymerized to form a composition 140 that includes aninsoluble polymer matrix 150 in a second solution 160. A major portionof the solvent 120 is removed from the second solution 160 to form thenanovoided article 170. The second solution 160 is depleted of thepolymerizable material 130; however, some polymerizable material 130 canremain in the second solution 160, as described elsewhere. Nanovoidedarticle 170 includes the insoluble polymer matrix 150 and a plurality ofvoids 180 having an average effective diameter 190. Although not shownin FIG. 1, it is to be understood that the first solution 110 can becoated on a substrate (not shown), to form a nanovoided coating on thesubstrate.

Polymerizable material 130 can be any polymerizable material that can bepolymerized by various conventional cationic or free radicalpolymerization techniques, which can be chemical, thermal, or radiationinitiated, including, e.g., solvent polymerization, emulsionpolymerization, suspension polymerization, bulk polymerization, andradiation polymerization, including, e.g., processes using actinicradiation including, e.g., visible and ultraviolet light, electron beamradiation and combinations thereof.

Actinic radiation curable materials include monomers, oligomers, andpolymers of acrylates, methacrylates, urethanes, epoxies and the like.Representative examples of energy curable groups suitable in thepractice of the present disclosure include epoxy groups, (meth)acrylategroups, olefinic carbon-carbon double bonds, allyloxy groups,alpha-methyl styrene groups, (meth)acrylamide groups, cyanate estergroups, vinyl ethers groups, combinations of these, and the like. Freeradically polymerizable groups are preferred. In some embodiments,exemplary materials include acrylate and methacrylate monomers, and inparticular, multifunctional monomers that can form a crosslinked networkupon polymerization can be used, as known in the art. The polymerizablematerials can include any mixture of monomers, oligomers and polymers;however the materials must be at least partially soluble in at least onesolvent. In some embodiments, the materials should be soluble in thesolvent monomer mixture.

As used herein, the term “monomer” means a relatively low molecularweight material (i.e., having a molecular weight less than about 500g/mole) having one or more energy polymerizable groups. “Oligomer” meansa relatively intermediate molecular weight material having a molecularweight of from about 500 up to about 10,000 g/mole. “Polymer” means arelatively high molecular weight material having a molecular weight ofat least about 10,000 g/mole, preferably at 10,000 to 100,000 g/mole.The term “molecular weight” as used throughout this specification meansnumber average molecular weight unless expressly noted otherwise.

Exemplary monomeric polymerizable materials include styrene,alpha-methylstyrene, substituted styrene, vinyl esters, vinyl ethers,N-vinyl-2-pyrrolidone, (meth)acrylamide, N-substituted (meth)acrylamide,octyl (meth)acrylate, iso-octyl (meth)acrylate, nonylphenol ethoxylate(meth)acrylate, isononyl (meth)acrylate, diethylene glycol(meth)acrylate, isobornyl (meth)acrylate, 2-(2-ethoxyethoxy)ethyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, lauryl (meth)acrylate,butanediol mono(meth)acrylate, beta-carboxyethyl (meth)acrylate,isobutyl (meth)acrylate, cycloaliphatic epoxide, alpha-epoxide,2-hydroxyethyl (meth)acrylate, (meth)acrylonitrile, maleic anhydride,itaconic acid, isodecyl (meth)acrylate, dodecyl (meth)acrylate, n-butyl(meth)acrylate, methyl (meth)acrylate, hexyl (meth)acrylate,(meth)acrylic acid, N-vinylcaprolactam, stearyl (meth)acrylate, hydroxyfunctional polycaprolactone ester (meth)acrylate, hydroxyethyl(meth)acrylate, hydroxymethyl (meth)acrylate, hydroxypropyl(meth)acrylate, hydroxyisopropyl (meth)acrylate, hydroxybutyl(meth)acrylate, hydroxyisobutyl (meth)acrylate, tetrahydrofurfuryl(meth)acrylate, combinations of these, and the like.

Oligomers and polymers may also be collectively referred to herein as“higher molecular weight constituents or species.” Suitable highermolecular weight constituents may be incorporated into compositions ofthe present disclosure. Such higher molecular weight constituents mayprovide benefits including viscosity control, reduced shrinkage uponcuring, durability, flexibility, adhesion to porous and nonporoussubstrates, outdoor weatherability, and/or the like. The amount ofoligomers and/or polymers incorporated into fluid compositions of thepresent disclosure may vary within a wide range depending upon suchfactors as the intended use of the resultant composition, the nature ofthe reactive diluent, the nature and weight average molecular weight ofthe oligomers and/or polymers, and the like. The oligomers and/orpolymers themselves may be straight-chained, branched, and/or cyclic.Branched oligomers and/or polymers tend to have lower viscosity thanstraight-chain counterparts of comparable molecular weight.

Exemplary polymerizable oligomers or polymers include aliphaticpolyurethanes, acrylics, polyesters, polyimides, polyamides, epoxypolymers, polystyrene (including copolymers of styrene) and substitutedstyrenes, silicone containing polymers, fluorinated polymers,combinations of these, and the like. For some applications, polyurethaneand acrylic-containing oligomers and/or polymers can have improveddurability and weatherability characteristics. Such materials also tendto be readily soluble in reactive diluents formed from radiationcurable, (meth)acrylate functional monomers.

Because aromatic constituents of oligomers and/or polymers generallytend to have poor weatherability and/or poor resistance to sunlight,aromatic constituents can be limited to less than 5 weight percent,preferably less than 1 weight percent, and can be substantially excludedfrom the oligomers and/or polymers and the reactive diluents of thepresent disclosure. Accordingly, straight-chained, branched and/orcyclic aliphatic and/or heterocyclic ingredients are preferred forforming oligomers and/or polymers to be used in outdoor applications.

Suitable radiation curable oligomers and/or polymers for use in thepresent disclosure include, but are not limited to, (meth)acrylatedurethanes (i.e., urethane (meth)acrylates), (meth)acrylated epoxies(i.e., epoxy (meth)acrylates), (meth)acrylated polyesters (i.e.,polyester (meth)acrylates), (meth)acrylated (meth)acrylics,(meth)acrylated silicones, (meth)acrylated polyethers (i.e., polyether(meth)acrylates), vinyl (meth)acrylates, and (meth)acrylated oils.

Solvent 120 can be any solvent that forms a solution with the desiredpolymerizable material 130. The solvent can be a polar or a non-polarsolvent, a high boiling point solvent or a low boiling point solvent,and a mixture of several solvents may be preferred. The solvent orsolvent mixture may be selected so that the insoluble polymer matrix 150formed is at least partially insoluble in the solvent (or at least oneof the solvents in a solvent mixture). In some embodiments, the solventmixture can be a mixture of a solvent and a non-solvent for thepolymerizable material. During polymerization, the first solution 110separates to form the second solution 160 and a polymer-rich solutionthat polymerizes to form the insoluble polymer matrix 150. In oneparticular embodiment, the insoluble polymer matrix 150 can be athree-dimensional polymer matrix having polymer chain linkages 155 thatprovide the three-dimensional framework. The polymer chain linkages 155can prevent deformation of the insoluble polymer matrix 150 afterremoval of the solvent 120.

In some embodiments, the second solution 160 can include some remainingpolymerizable material 135 that is not incorporated in the insolublepolymer matrix 150, as shown in FIG. 1 (i.e., the second solution 160has become depleted of polymerizable material 135, but some may still bepresent). It is preferred to minimize the amount of remainingpolymerizable material 135 in the second solution 160, by maximizing theextent of polymerization of composition 140.

In one embodiment, solvent 120 can be easily removed from thecomposition 140 by drying, for example, at temperatures not exceedingthe decomposition temperature of either the insoluble polymer matrix150, or the substrate (if included). In one particular embodiment, thetemperature during drying is kept below a temperature at which thesubstrate is prone to deformation, e.g., a warping temperature or aglass-transition temperature of the substrate. Exemplary solventsinclude linear, branched, and cyclic hydrocarbons, alcohols, ketones,and ethers, including for example, propylene glycol ethers such asDOWANOL™ PM propylene glycol methyl ether, isopropyl alcohol, ethanol,toluene, ethyl acetate, 2-butanone, butyl acetate, methyl isobutylketone, water, methyl ethyl ketone, cyclohexanone, acetone, aromatichydrocarbons, isophorone, butyrolactone, N-methylpyrrolidone,tetrahydrofuran, esters such as lactates, acetates, propylene glycolmonomethyl ether acetate (PM acetate), diethylene glycol ethyl etheracetate (DE acetate), ethylene glycol butyl ether acetate (EB acetate),dipropylene glycol monomethyl acetate (DPM acetate), iso-alkyl esters,isohexyl acetate, isoheptyl acetate, isooctyl acetate, isononyl acetate,isodecyl acetate, isododecyl acetate, isotridecyl acetate or otheriso-alkyl esters; combinations of these and the like.

The first solution 110 can also include other ingredients including,e.g., initiators, curing agents, cure accelerators, catalysts,crosslinking agents, tackifiers, plasticizers, dyes, surfactants, flameretardants, coupling agents, pigments, impact modifiers includingthermoplastic or thermoset polymers, flow control agents, foamingagents, fillers, glass and polymer microspheres and microparticles,other particles including electrically conductive particles, thermallyconductive particles, fibers, antistatic agents, antioxidants, UVabsorbers, and the like.

An initiator, such as a photoinitiator, can be used in an amounteffective to facilitate polymerization of the monomers present in thefirst solution 110. The amount of photoinitiator can vary dependingupon, for example, the type of initiator, the molecular weight of theinitiator, the intended application of the resulting insoluble polymermatrix 150 and the polymerization process including, e.g., thetemperature of the process and the wavelength of the actinic radiationused. Useful photoinitiators include, for example, those available fromCiba Specialty Chemicals under the IRGACURE™ and DAROCURE™ tradedesignations, including IRGACURE™ 184 and IRGACURE™ 819.

In some embodiments, a mixture of initiators and initiator types can beused, for example to control the polymerization in different sections ofthe process. In one embodiment, optional post-processing polymerizationmay be a thermally initiated polymerization that requires a thermallygenerated free-radical initiator. In other embodiments, optionalpost-processing polymerization may be an actinic radiation initiatedpolymerization that requires a photoinitiator. The post-processingphotoinitiator may be the same or different than the photoinitiator usedto polymerize the polymer matrix in solution.

The insoluble polymer matrix 150 may be cross-linked to provide a morerigid polymer network. Cross-linking can be achieved with or without across-linking agent by using high energy radiation such as gamma orelectron beam radiation. In some embodiments, a cross-linking agent or acombination of cross-linking agents can be added to the mixture ofpolymerizable monomers. The cross-linking can occur duringpolymerization of the polymer network using any of the actinic radiationsources described elsewhere.

Useful radiation curing cross-linking agents include multifunctionalacrylates and methacrylates, such as those disclosed in U.S. Pat. No.4,379,201 (Heilmann et al.), which include 1,6-hexanedioldi(meth)acrylate, trimethylolpropane tri(meth)acrylate, 1,2-ethyleneglycol di(meth)acrylate, pentaerythritol tri/tetra(meth)acrylate,triethylene glycol di(meth)acrylate, ethoxylated trimethylolpropanetri(meth)acrylate, glycerol tri(meth)acrylate, neopentyl glycoldi(meth)acrylate, tetraethylene glycol di(meth)acrylate, 1,12-dodecanoldi(meth)acrylate, copolymerizable aromatic ketone co-monomers such asthose disclosed in U.S. Pat. No. 4,737,559 (Kellen et al.) and the like,and combinations thereof.

The first solution 110 may also include a chain transfer agent. Thechain transfer agent is preferably soluble in the monomer mixture priorto polymerization. Examples of suitable chain transfer agents includetriethyl silane and mercaptans. In some embodiments, chain transfer canalso occur to the solvent; however this may not be a preferredmechanism.

The polymerizing step preferably includes using a radiation source in anatmosphere that has a low oxygen concentration. Oxygen is known toquench free-radical polymerization, resulting in diminished extent ofcure. The radiation source used for achieving polymerization and/orcrosslinking may be actinic (e.g., radiation having a wavelength in theultraviolet or visible region of the spectrum), accelerated particles(e.g., electron beam radiation), thermal (e.g., heat or infraredradiation), or the like. In some embodiments, the energy is actinicradiation or accelerated particles, because such energy providesexcellent control over the initiation and rate of polymerization and/orcrosslinking Additionally, actinic radiation and accelerated particlescan be used for curing at relatively low temperatures. This avoidsdegrading or evaporating components that might be sensitive to therelatively high temperatures that might be required to initiatepolymerization and/or crosslinking of the energy curable groups whenusing thermal curing techniques. Suitable sources of curing energyinclude UV LEDs, visible LEDs, lasers, electron beams, mercury lamps,xenon lamps, carbon arc lamps, tungsten filament lamps, flashlamps,sunlight, low intensity ultraviolet light (black light), and the like.

A major portion of the solvent 120 is removed in the solvent removalstep to produce the nanovoided article 170. By a major portion of thesolvent is meant greater than 90%, 80%, 70%, 60%, or greater than 50% byweight of the solvent. Solvent can be removed by drying in a thermaloven that can include air floatation/convection, drying with infrared orother radiant light sources, vacuum drying, gap drying, or a combinationof drying techniques. The choice of drying technique can be governed bythe desired process speed, extent of solvent removal, and expectedcoating morphology, among others. In one particular embodiment, gapdrying can offer advantages for solvent removal, as gap drying offersrapid drying within minimal space.

FIG. 2 shows a schematic view of a process 200 for forming a nanovoidedarticle 280 according to another aspect of the disclosure. A firstsolution 210 that includes a polymerizable material 230 andnanoparticles 240 in a solvent 220 is provided. The first solution 210is at least partially polymerized to form a composition 250 includingthe nanoparticles 240 bound to an insoluble polymer matrix 260 in asecond solution 270. A major portion of the solvent 220 is removed fromthe second solution 270 to form the nanovoided article 280. In oneparticular embodiment, the insoluble polymer matrix 260 can be athree-dimensional polymer matrix having polymer chain linkages 265 thatprovide the three-dimensional framework. The polymer chain linkages 265can prevent deformation of the insoluble polymer matrix 260 afterremoval of the solvent 220.

In some embodiments, the second solution 270 can include some remainingpolymerizable material 235 that is not incorporated in the insolublepolymer matrix 260, as shown in FIG. 2 (i.e., the second solution 270has become depleted of polymerizable material 235, but some may still bepresent). It is preferred to minimize the amount of remainingpolymerizable material 235 in the second solution 270, after thepolymerizing step. In some embodiments, the second solution 270 can alsoinclude a minor portion of nanoparticles 245 that are not bound to theinsoluble polymer matrix 260, as shown in FIG. 2 (i.e., the secondsolution 270 has become depleted of nanoparticles 240, but some maystill be present). It is generally desired to minimize the quantity ofnanoparticles 245 that are not bound to the insoluble polymer matrix 260after the polymerizing step. As used herein, nanoparticles “bound to”the polymer matrix is meant to include nanoparticles completely embeddedin the polymer matrix, nanoparticles partially embedded in the polymermatrix, nanoparticles attached to the surface of the polymer matrix, ora combination thereof.

In one particular embodiment, nanoparticles 240 can be surface modifiedreactive nanoparticles that are chemically bound to the insolublepolymer matrix 260. In one particular embodiment, nanoparticles 240 canbe surface modified non-reactive nanoparticles that are physically boundto the insoluble polymer matrix 260. In one particular embodiment,nanoparticles 240 can be a mixture of surface modified reactive andnon-reactive nanoparticles.

Nanovoided article 280 includes the nanoparticles 240 bound to theinsoluble polymer matrix 260, and a plurality of voids 290 having anaverage effective diameter 295. Although not shown in FIG. 2, it is tobe understood that the first solution 210 can be coated on a substrateto form a nanovoided coating on the substrate.

The polymerizable material 230 and solvent 220 can be the same asdescribed for polymerizable material 130 and solvent 120, respectively,of FIG. 1. In one embodiment, the nanoparticles 240 can be inorganicnanoparticles, organic (e.g., polymeric) nanoparticles, or a combinationof organic and inorganic nanoparticles. In one particular embodiment,particles 240 can be porous particles, hollow particles, solidparticles, or a combination thereof. Examples of suitable inorganicnanoparticles include silica and metal oxide nanoparticles includingzirconia, titania, ceria, alumina, iron oxide, vanadia, antimony oxide,tin oxide, alumina/silica, and combinations thereof. The nanoparticlescan have an average particle diameter less than about 1000 nm, less thanabout 100 nm, less than about 50 nm, or from about 3 nm to about 50 nm.In some embodiments, the nanoparticles can have an average particlediameter from about 3 nm to about 50 nm, or from about 3 nm to about 35nm, or from about 5 to about 25 nm. If the nanoparticles are aggregated,the maximum cross sectional dimension of the aggregated particle can bewithin any of these ranges, and can also be greater than about 100 nm.In some embodiments, “fumed” nanoparticles, such as silica and alumina,with primary size less than about 50 nm, are also included, such asCAB-O-SPERSE® PG 002 fumed silica, CAB-O-SPERSE® 2017A fumed silica, andCAB-O-SPERSE® PG 003 fumed alumina, available from Cabot Co. Boston,Mass.

In some embodiments, the nanoparticles 240 include surface groupsselected from the group consisting of hydrophobic groups, hydrophilicgroups, and combinations thereof. In other embodiments, thenanoparticles include surface groups derived from an agent selected fromthe group consisting of a silane, organic acid, organic base, andcombinations thereof. In other embodiments, the nanoparticles includeorganosilyl surface groups derived from an agent selected from the groupconsisting of alkylsilane, arylsilane, alkoxysilane, and combinationsthereof.

The term “surface-modified nanoparticle” refers to a particle thatincludes surface groups attached to the surface of the particle. Thesurface groups modify the character of the particle. The terms “particlediameter” and “particle size” refer to the maximum cross-sectionaldimension of a particle. If the particle is present in the form of anaggregate, the terms “particle diameter” and “particle size” refer tothe maximum cross-sectional dimension of the aggregate. In someembodiments, particles can be large aspect ratio aggregates ofnanoparticles, such as fumed silica particles.

The surface-modified nanoparticles have surface groups that modify thesolubility characteristics of the nanoparticles. The surface groups aregenerally selected to render the particle compatible with the firstsolution 210. In one embodiment, the surface groups can be selected toassociate or react with at least one component of the first solution210, to become a chemically bound part of the polymerized network.

A variety of methods are available for modifying the surface ofnanoparticles including, e.g., adding a surface modifying agent tonanoparticles (e.g., in the form of a powder or a colloidal dispersion)and allowing the surface modifying agent to react with thenanoparticles. Other useful surface modification processes are describedin, e.g., U.S. Pat. No. 2,801,185 (Iler) and U.S. Pat. No. 4,522,958(Das et al.), and incorporated herein.

Useful surface-modified silica nanoparticles include silicananoparticles surface-modified with silane surface modifying agentsincluding, e.g., Silquest® silanes such as Silquest® A-1230 from GESilicones, 3-acryloyloxypropyl trimethoxysilane,3-methacryloyloxypropyltrimethoxysilane,3-mercaptopropyltrimethoxysilane, n-octyltrimethoxysilane,isooctyltrimethoxysilane, 4-(triethoxysilyl)-butyronitrile,(2-cyanoethyl)triethoxysilane,N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate (PEG3TMS),N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate (PEG2TMS),3-(methacryloyloxy)propyltriethoxysilane,3-(methacryloyloxy)propylmethyldimethoxysilane,3-(acryloyloxypropyl)methyldimethoxysilane,3-(methacryloyloxy)propyldimethylethoxysilane,3-(methacryloyloxy)propyldimethylethoxysilane,vinyldimethylethoxysilane, phenyltrimethoxysilane,n-octyltrimethoxysilane, dodecyltrimethoxysilane,octadecyltrimethoxysilane, propyltrimethoxysilane,hexyltrimethoxysilane, vinylmethyldiacetoxysilane,vinylmethyldiethoxysilane, vinyltriacetoxysilane, vinyltriethoxysilane,vinyltriisopropoxysilane, vinyltrimethoxysilane, vinyltriphenoxysilane,vinyltri-t-butoxysilane, vinyltris-isobutoxysilane,vinyltriisopropenoxysilane, vinyltris(2-methoxyethoxy)silane, andcombinations thereof. Silica nanoparticles can be treated with a numberof surface modifying agents including, e.g., alcohol, organosilaneincluding, e.g., alkyltrichlorosilanes, trialkoxyarylsilanes,trialkoxy(alkyl)silanes, and combinations thereof and organotitanatesand mixtures thereof.

The nanoparticles may be provided in the form of a colloidal dispersion.Examples of useful commercially available unmodified silica startingmaterials include nano-sized colloidal silicas available under theproduct designations NALCO 1040, 1050, 1060, 2326, 2327, and 2329colloidal silica from Nalco Chemical Co., Naperville, Ill.; theorganosilica under the product name IPA-ST-MS, IPA-ST-L, IPA-ST,IPA-ST-UP, MA-ST-M, and MA-ST sols from Nissan Chemical America Co.Houston, Tex. and the SnowTex® ST-40, ST-50, ST-20L, ST-C, ST-N, ST-O,ST-OL, ST-ZL, ST-UP, and ST-OUP, also from Nissan Chemical America Co.Houston, Tex. The weight ratio of polymerizable material tonanoparticles can range from about 30:70, 40:60, 50:50, 55:45, 60:40,70:30, 80:20 or 90:10 or more. The preferred ranges of wt % ofnanoparticles range from about 10% by weight to about 60% by weight, andcan depend on the density and size of the nanoparticle used.

FIG. 3A shows a schematic view of a process 300 for forming a nanovoidedcoating 356 on a substrate 302, according to one aspect of thedisclosure. The process 300 shown in FIG. 3A is a continuous process,although it is to be understood that the process can instead beperformed in a stepwise manner, i.e., the steps of coating,polymerizing, and removing solvent described below can be performed onindividual substrate pieces in discrete operations, to form thenanovoided coating.

The process 300 shown in FIG. 3A passes a substrate 302 through acoating section 310, an optional coating conditioning section 315, apolymerization section 320, a first solvent removal section 340, and anoptional second solvent removal section 350 to form the nanovoidedcoating 356 on the substrate 302. Nanovoided coating 356 on substrate302 then passes through optional second polymerization section 360 toform an optionally post-cured nanovoided coating 366 on the substrate302, which is then wound up as an output roll 370. In some embodiments,process 300 can include additional processing equipment common to theproduction of web-based materials, including, for example, idler rolls;tensioning rolls; steering mechanisms; surface treaters such as coronaor flame treaters; lamination rolls; and the like. In some embodiments,the process 300 can utilized different web paths, coating techniques,polymerization apparatus, positioning of polymerization apparatus,drying ovens, conditioning sections, and the like, and some of thesections described can be optional.

The substrate 302 can be any known substrate suitable for roll-to-rollweb processing in a webline, including, for example, polymericsubstrates, metalized polymeric substrates, metal foils, combinationsthereof, and the like. In one particular embodiment, the substrate 302is an optical quality polymeric substrate, suitable for use in anoptical display such as a liquid crystal display.

The substrate 302 is unwound from an input roll 301, passes over idlerrolls 303 and contacts coating roll 304 in coating section 310. A firstsolution 305 passes through a coating die 307 to form a first coating306 of first solution 305 on substrate 302. The first solution 305 caninclude solvents, polymerizable materials, optional nanoparticles,photoinitiators, and any of the other first solution componentsdescribed elsewhere. A shroud 308 positioned between the coating die 307in the coating section 310, and a coating conditioning region 309 in theoptional coating conditioning section 315, can provide a firstcontrolled environment 311 surrounding the first solution 305. In someembodiments, the shroud 308 and optional coating conditioning section315 can be optional, for example, when the polymerization occurs beforesubstantial change can occur in the composition of the first solution305. The substrate 302 having the first coating 306 of first solution305 then enters the polymerization section 320 where the first solution305 is polymerized, as described elsewhere.

The coating die 307 can include any known coating die and coatingtechnique, and is not to be limited to any specific die design ortechnique of coating thin films. Examples of coating techniques includeknife coating, gravure coating, slide coating, slot coating, slot-fedknife coating, curtain coating, and the like as known to those skilledin the art. Several applications of the nanovoided article can includethe need for precise thickness and defect-free coatings, and may requirethe use of a precise slot coating die 307 positioned against a precisioncoating roll 304 as shown in FIG. 3A. The first coating 306 can beapplied at any thickness; however thin coatings are preferred, forexample coatings less than 1000 microns thick, less than about 500microns thick, or even less than about 100 microns thick can providenanovoided articles having exemplary properties.

Because the first coating 306 includes at least one solvent and apolymerizable material as described elsewhere, the shroud 308 ispositioned to reduce any undesired loss of solvent from the coating, andalso to protect the coating from oxygen which can inhibit thepolymerization. The shroud 308 can be, for example, a formed aluminumsheet that is positioned in close proximity to the first coating 306 andprovides a seal around the coating die 307 and the coating roll 304 sothat the first controlled environment 311 can be maintained. In someembodiments, the shroud 308 can also serve to protect the coating fromambient room conditions. The first controlled environment 311 caninclude inerting gases such as nitrogen to control oxygen content,solvent vapors to reduce the loss of solvent, or a combination of inertgases and solvent vapors. The oxygen concentration can affect both therate and extent of polymerization, so in one embodiment, the oxygenconcentration in the first controlled environment 311 is reduced to lessthan 1000 parts-per-million (ppm), less than 500 ppm, less than 300 ppm,less than 150 ppm, less than 100 ppm, or even less than 50 ppm.Generally, the lowest oxygen concentration that can be attained ispreferred.

The coating conditioning region 309 in the optional coating conditioningsection 315 is an extension of the shroud 308 that provides additionalcapabilities to modify the first coating 306 before entering thepolymerization section 320. The first controlled environment 311 canstill be maintained within coating conditioning region 309. In otherembodiments, additional heating, cooling, or input and exhaust gases canbe provided to adjust or maintain the composition of the first coating306. For example, solvent vapor can be introduced in the input gas toreduce evaporation of solvent from the first coating 306 prior topolymerization.

A heating apparatus, such as a gap dryer described, for example, in U.S.Pat. No. 5,694,701 can be used to raise or lower the temperature offirst coating 306, drive off additional solvent to adjust thecomposition of first coating 306, or both. The gap dryer could also beused to remove a portion of the solvent before the polymerizationsection to enable the desired thin film morphology, for example when theoptimum composition of the coating (e.g., % solids) is different fromthe optimum composition for polymerization. Often, coating conditioningregion 309 can serve to provide additional time for the first coating306 to stabilize, for example, to smooth any surface ripples or streaks,prior to polymerization.

FIG. 3B is a schematic view of the polymerization section 320 of process300 shown in FIG. 3A, according to one aspect of the disclosure. FIG. 3Bshows a cross-section of the polymerization section 320 as viewed downthe path of the substrate 302. Polymerization section 320 includes ahousing 321 and a quartz plate 322 that provide boundaries of a secondcontrolled environment 327 that partially surrounds the first coating306 on substrate 302. A radiation source 323 generates actinic radiation324 that passes through quartz plate 322 and polymerizes the firstcoating 306 on substrate 302. Instead of a single radiation source 323,a radiation source array 325 shown in FIG. 3B can provide improveduniformity and rate of polymerization to the polymerization process. Theradiation source array 325 can provide individual control of radiationsource 323, for example, crossweb or downweb profiles can be generatedas desired. A heat extractor 326 can be positioned to control thetemperature by removing heat generated by each radiation source 323 inthe radiation source array 325.

The housing 321 can be a simple enclosure designed to surround thesubstrate 302, first coating 306, and an at least partially polymerizedsecond coating 336 (shown in FIG. 3C), or the housing 321 can alsoinclude additional elements, such as, for example, temperaturecontrolled plates (not shown) that can adjust the temperature of asecond controlled environment 327. The housing 321 has sufficientinterior dimensions “h3” and “h2” to enclose substrate 302 and firstcoating 306 to provide the second controlled environment 327. The gasflow fields impact inerting capabilities, coating composition, coatinguniformity and the like. As shown in FIG. 3B, the housing 321 includes atop quartz plate 322 separating the second controlled environment 327from radiation source 323 in radiation source array 325. The radiationsource array 325 is positioned a distance “h1” from the substrate 302 toprovide uniform actinic radiation 324 to the first coating 306. In oneembodiment, “h1” and “h3” are 1 inch (2.54 cm) and 0.25 inch (0.64 cm),respectively. In some embodiments (not shown), the polymerizationsection 320 can be inverted so that the quartz plate 322 and radiationsource 323 are located beneath the substrate 302, and actinic radiation324 passes through the substrate 302 before polymerizing first coating306. In other embodiments (also not shown), the polymerization section320 can include two quartz plates 322 and two radiation sources 323,located above and below the substrate, to polymerize first coating 306.

The radiation source 323 can be any source of actinic radiation asdescribed elsewhere. In some embodiments, radiation source 323 is anultraviolet LED that is capable of producing UV radiation. A combinationof radiation sources emitting at different wavelengths can be used tocontrol the rate and extent of the polymerization reaction. The UV-LEDsor other radiation sources can generate heat during operation, and theheat extractor 326 can be fabricated from aluminum that is cooled byeither air or water to control the temperature by removing the generatedheat.

FIG. 3C is a schematic view of the polymerization section 320 of process300 shown in FIG. 3A, according to one aspect of the disclosure. FIG. 3Cshows a cross-section of the polymerization section 320 as viewed alongan edge of the substrate 302. Polymerization section 320 includes thehousing 321 and the quartz plate 322 that provide boundaries of thesecond controlled environment 327. The second controlled environment 327partially surrounds the first coating 306 and the at least partiallypolymerized second coating 336 on substrate 302. At least partiallypolymerized second coating 336 includes an insoluble polymer matrix in asecond solution, as described elsewhere.

The second controlled environment 327 will now be described. Housing 321includes an entrance aperture 328 and an exit aperture 329 that can beadjusted to provide any desired gap between the substrate 302, thecoating 306 on substrate 302, and the respective aperture. The secondcontrolled environment 327 can be maintained by control of thetemperature of the housing 321, and appropriate control of thetemperature, composition, pressure and flow rate of a first input gas331, a second input gas 333, a first output gas 335 and a second outputgas 334. Appropriate adjustment of the sizes of the entrance and exitapertures 328, 329 can aid control of the pressure and flow rate of thefirst and second output gases 335, 334, respectively.

The first output gas 335 can flow from the second controlled environment327 through the entrance aperture 328 and into the first controlledenvironment 311 of optional coating conditioning section 315, shown inFIG. 3A. In some embodiments, the pressure within the second controlledenvironment 327 and the first controlled environment 311 are adjusted toprevent flow between the two environments, and first output gas 335 canexit second controlled environment 327 from another location (not shown)within housing 321. The second output gas 334 can flow from the secondcontrolled environment 327 through the exit aperture 329, and into thefirst solvent removal section 340 shown in FIG. 3A, or the second outputgas 334 can exit second controlled environment 327 from another location(not shown) within housing 321.

A first input gas manifold 330 is positioned adjacent the housing 321proximate the entrance aperture 328, to distribute the first input gas331 with desired uniformity across the width of the first coating 306. Asecond input gas manifold 332 is positioned adjacent the housing 321proximate the exit aperture 329, to distribute the second input gas 333with desired uniformity across the width of the second coating 336.First and second input gases 331, 333 can be distributed above the web,below the web, or in any combination of above and below the web, asdesired. First and second input gases 331, 333 can be the same or theycan be different, and can include inerting gasses such as nitrogen,which can reduce oxygen concentration that can inhibit thepolymerization reaction, as is known. First and second input gases 331,333 can also include solvent vapors that can help reduce the loss ofsolvent from first coating 306 before or during polymerization, asdescribed elsewhere. The relative flow rates, flow velocities, flowimpingement or orientation on the coating, and temperature of each ofthe first and second input gases 331, 333 can be controlledindependently, and can be adjusted to reduce imperfections in the firstcoating 306 prior to polymerization. The imperfections can be caused bydisturbances to the coating, as known in the art. In some cases, onlyone of the first and second input gases 331, 333 may be flowing.

Returning now to FIG. 3A, the remainder of the process will bedescribed. After leaving polymerization section 320, second polymerizedcoating 336 on substrate 302 enters first solvent removal section 340.First solvent removal section 340 can be a conventional drying oven thatremoves solvent by heating the second polymerized coating 336 toevaporate the solvent. A preferred first solvent removal section 340 isa gap dryer, such as described, for example, in U.S. Pat. Nos. 5,694,701and 7,032,324. A gap dryer can provide greater control of the dryingenvironment, which may be desired in some applications. An optionalsecond solvent removal section 350 can further be used to ensure that amajor portion of the solvent is removed.

A nanovoided coating 356 on substrate 302 exits optional second solventremoval section 350 and then passes through optional secondpolymerization section 360 to form an optionally post-cured nanovoidedcoating 366 on the substrate 302. Optional second polymerization section360 can include any of the actinic radiation sources previouslydescribed, and can increase the extent of cure of the nanovoided coating356. In some embodiments, increasing the extent of cure can includepolymerizing remaining polymerizable material (i.e., remainingpolymerizable material 135, 235, shown in FIG. 1 and FIG. 2,respectively) after removal of the solvent. Nanovoided coating 356 onsubstrate 302 exits optional second polymerization section 360 and isthen wound up as an output roll 370. In some embodiments, output roll370 can have other desired films (not shown) laminated to the nanovoidedcoating and simultaneously wound on the output roll 370. In otherembodiments, additional layers (not shown) can be coated, cured, anddried on either the nanovoided coating 356 or the substrate 302.

EXAMPLES

The following list of materials and their source is referred tothroughout the Examples.

Nalco 2327 - colloidal silica Nalco Co. Naperville IL dispersion3-(Methacryloyloxy)propyltrimethoxy Aldrich Chemical, Milwaukee WIsilane Silquest ® A-174 silane GE Advanced Materials, Wilton CT1-methoxy-2-propanol - solvent Aldrich Chemical, Milwaukee WI Prostabb5128 - hindered amine Ciba Specialties Chemical, nitroxide inhibitorTarrytown NY SR444 Pentaerythritol triacrylate Sartomer Company, ExtonPA SR238 Hexanedioldiacrylate Sartomer Company, Exton PA SR506 IsobornylAcrylate Sartomer Company, Exton PA Irgacure 184 - photoinitiator CibaSpecialties Chemical, Tarrytown NY Irgacure 819 - photoinitiator CibaSpecialties Chemical, Tarrytown NY Tinuvin 292 - hindered amine lightCiba Specialties Chemical, stabilizer Tarrytown NY ethyl acetate -solvent Aldrich Chemical, Milwaukee WI IPA - isopropyl alcohol (solvent)Aldrich Chemical, Milwaukee WI DOWANOL ™ PM glycol ether - Dow Chemical,Midland MI solvent

Example 1 Nanoporous Article Formed by Polymerization in Solution

A first coating solution including surface modified nanoparticles wasprepared. The surface modified nanoparticles were reactive nanoparticlesthat would chemically bond with the polymer during polymerization.

Preparation of Reactive Nanoparticles:

400 g of a Nalco 2327 colloidal silica dispersion was charged to a 1quart jar. In a separate container, 450 g of 1-methoxy-2-propanol, 25.43g of 3-(Methacryloyloxy)propyltrimethoxy silane and 0.27 g of Prostab5128 (1% by weight in water) were mixed together and added to thecolloidal silica dispersion while stirring. The jar was sealed andheated to 80° C. for 17 hours, resulting in a solution of reactivesurface modified nanoparticles.

Preparation of Radiation Curable Hard Coat (RCHC) Coating Solution:

Three acrylate monomers, SR444 (400 g), SR238 (400 g) and SR506 (200 g)were combined and mixed to form a radiation curable resin. In a separateflask, 667 g of the solution of reactive nanoparticles (above) was mixedwith 174 g of the radiation curable resin, and 1.22 g of Prostab 5128(5% by weight in water). The water and 1-methoxy-2-propanol were removedfrom the mixture via rotary evaporation. This yielded a clearcomposition (approximately 308 g) having a weight percent of modifiedsilica of about 43%. A first coating solution was prepared by dilutingthe clear composition to 40% solids using ethyl acetate, and addingIrgacure 819 photoinitiator at 2% by weight of solids.

Processing Coating Solution:

The general process followed the schematic presented in FIGS. 3A-C. Thefirst coating solution was supplied at a rate of 3 cc/min to a 4 inch(10.2 cm) wide slot-type coating die. The substrate was moving at aspeed of 10 ft/min (305 cm/min), resulting in a wet coating thickness ofapproximately 8 microns. The 4 inch wide coating die was inside aclamshell enclosure (i.e., shroud) and the clamshell was supplied withnitrogen at a flow rate of 180 cubic feet/hour (84 liters/min). Theclamshell was directly coupled to a small gap web enclosure providedwith two quartz windows. The nitrogen flow to the clamshell provided forinerting of the small gap polymerization section to a level of 360 ppmoxygen.

The polymerization section included two Clearstone Tech UV LED unitshaving 18 LEDs positioned within a 1.75 inch (4.4 cm) diameter circle,available from Clearstone Technologies Inc., Minneapolis Minn. The twoUV LED units were positioned directly over the quartz windows and whenturned ON, operated at 100% power. The wavelengths of each UV LED unitwere 405 nm and 365 nm, respectively. The 365 nm UV LED producedapproximately 0.11 W/cm² UV-A, and 0 W/cm² visible radiation, and the405 nm UV LED produced approximately 0.03 W/cm² UV-A, and 0.196 W/cm²visible radiation. The LEDs were powered by a CF 1000 UV-Vis LED Source,also available from Clearstone. Two samples were prepared, one with theUV LEDs turned on, and one with the UV LEDs turned off.

Following UV LED polymerization, the coated web traveled a 10 ft (3 m)span in the room environment, and then passed through two 5 ft (1.5 m)long zones of small gap drying with plate temperatures set at 170° F.(77° C.). The coating was then post-polymerized using a Fusion SystemsModel 1300P (Gaithersburg Md.) fitted with an H-bulb. The UV chamber wasnitrogen-inerted to approximately 50 ppm oxygen. FIG. 4A shows an SEM ofthe sample prepared with the UV LEDs turned off, and FIG. 4B shows anSEM of the sample prepared with the UV LEDs turned on.

Comparison of FIGS. 4A and 4B shows the ability of UV LED polymerizationto change the thin film morphology. FIG. 4A shows that withoutpolymerization while in solution, the coating is non-porous. FIG. 4Bshows that with polymerization while in solution, a nanovoided coatingwith pore dimensions in the nanometer size range, results.

The refractive index of each of the samples was measured at 632.8 nmusing a Metricon MODEL 2010 prism coupler (Metricon Corporation Inc.,Pennington, N.J.). The surface roughness of the samples was measuredusing Digital Instruments Dimension 5000 SPM AFM System. The volumefraction of air was estimated using the volume average relationshipbetween refractive index and volume fraction; V₁n₁+V₂n₂=n_(f) andV₁+V₂=1. Here V₁ is the volume fraction of air, n₁ is the refractiveindex of air, V₂ is the volume fraction of resin/particle, n₂ is therefractive index of the resin/particle, and n_(f) is the refractiveindex of the resulting coating. Measuring the refractive index of theresulting coating and knowing the refractive index of air andresin/particle system, the equations can be used to estimate the volumefraction of air in the film. The results are summarized in Table 1below.

TABLE 1 Calculated UV Refractive Volume Fraction RMS Surface LED Indexof Air Roughness (nm) Off 1.49 0 3.33 On 1.37 0.23 15.9

Example 2 Control of Nanoporosity and Refractive Index

A solution of a radiation curable material having silica nanoparticles(906 Hardcoat solution) was used to generate thin films polymerizedusing a range of UV LED power levels. A 906 Hardcoat stock solution madeby the process described in column 10, line 25-39 and example 1 of U.S.Pat. No. 5,677,050 to Bilkadi et al., is a composition that includes:18.4 wt % 20 nm silica (Nalco 2327) surface modified withmethacryloyloxypropyltrimethoxysilane (acrylate silane), 25.5 wt %pentaerthritol tri/tetra acrylate (PETA), 4.0 wt %N,N-dimethylacrylamide (DMA), 1.2 wt % Irgacure 184, 1.0 wt % Tinuvin292, 46.9 wt % isopropanol, and 3.0 wt % water. The stock solution of906 Hardcoat was diluted to 35% solids with 1-methoxy 2-propanol to forma first coating solution. The first coating solution was applied to a0.002 inch (0.051 mm) thick primed polyester (Melinex 617, DuPont TeijinFilms) substrate web, in the process shown in FIGS. 3A-3C.

The first coating solution was supplied at a rate of 3 cc/min to a 4inch (10.2 cm) wide slot type coating die, onto a substrate moving at aspeed of 5 ft/min (152 cm/min), resulting in a wet coating thickness ofapproximately 19 microns. After coating, the web passed through a shroudto reduce evaporation of the solvent before entering the UV LEDpolymerization section. The UV LED radiation source array had 8 LEDsacross the width of the web and 20 rows of LEDs down the length of theweb in a 4 inch (10.2 cm) by 8 inch (20.4 cm) area. The LEDs were 385 nmUV LEDs (available from Cree Inc., Durham N.C.). The UV LED array wasspaced approximately 1 inch (2.54 cm) from the substrate with the 4″(10.2 cm) dimension positioned in the downweb direction. The fan-cooledarray is powered by a TENMA 72-6910 (42V/10 A) power supply (availablefrom TENMA, Centerville Ohio). The power supply output was controlledfrom 0 to 8 amps. The UV LED polymerization section was supplied with100 cubic feet/hour (46.7 liters/min) of nitrogen from a downstream(i.e. manifold 332 in FIG. 3C) gas introduction device which resulted inapproximately 150 ppm oxygen concentration in the controlledenvironment.

Following the UV LED polymerization, the coated web passed a 10 ft (3 m)span in the room environment, and then passed through two 5 ft (1.5 m)long sections of small gap drying with plate temperatures set at 170° F.(77° C.). The coating was then post-polymerized using a Fusion SystemsModel 1300P (Gaithersburg Md.) fitted with an H-bulb. The UV chamber wasnitrogen-inerted to approximately 50 ppm oxygen.

Nine samples were prepared to demonstrate control of the nanoporosityand the coating refractive index, by varying the UV LED power supplyfrom 0 to 8 amps in 1 amp increments. The refractive indices of the 9coatings were measured at 632.8 nm using a Metricon MODEL 2010 prismcoupler (Metricon Corporation Inc., Pennington, N.J.), and the resultswere plotted in FIG. 5. With power levels less than about 1 amp, thefinal coating morphology did not change and the refractive index(n=1.49) was that expected of 906 Hardcoat after sequential coating,drying, and curing. With increasing power level, the refractive indexprogressively drops reaching a minimum value (n=1.22) at a power settingof 8 amps.

Using the volume average relationship presented in Example 1, thecalculated volume fraction of air in the film having the measuredrefractive of 1.22 is 0.56. This example shows the amount of air in thecoating can be varied from 0 to 56%, by controlling the polymerizationconditions.

Example 3 Demonstration of High Volume Low Cost Roll-to-RollManufacturing

The 35% solids 906 Hardcoat solution from Example 2 was further dilutedto 30% solids with the addition of ethyl acetate. Irgacure 819 was thenadded at 2% by weight of solids. The coating solution was applied to a0.002 inch (0.051 mm) thick primed polyester (Melinex 617, DuPont TeijinFilms) substrate web, in the process shown in FIGS. 3A-3C.

The first coating solution was supplied to an 8 inch (20.3 cm) wide slottype coating die, onto a web moving at a speed that was varied from 20to 100 ft/min (6.1 to 30.5 m/min). The rate of application of thecoating solution was increased as the web speed was increased tomaintain a constant wet coating thickness of 19 microns. After coating,the web passed through a web enclosure (i.e., shroud 308 in FIG. 3A)before entering a 5 ft (152 cm) long section of Gap dryer (correspondsto the coating conditioning region 309 in FIG. 3A). The Gap dryer wasoperating with a 0.25 inch (0.64 cm) gap and both upper and lower platesset at 70° F. (21° C.), conditions set to minimize drying between thecoating die and polymerization section. The UV LED polymerizationapparatus was directly coupled with the downweb end of the Gap dryer.

The coated web then passed into the polymerization section which used a395 nm UV LED water-cooled array consisting of 16 rows of LEDs with 22LEDs in each row. The 22 LEDs in each row were equally spaced across theweb width, and the 16 rows were equally spaced along the downwebdirection in an area of 8″×8″ (20.3×20.3 cm). The 352 LEDs in the arraywere 395 nm UV LEDs (available from Cree Inc., Durham N.C.). The LEDarray was powered using a LAMBDA GENH750 W power supply. The powersupply output can be varied from 0 to 13 amps and operated atapproximately 45 volts. The controlled environment was supplied withapproximately 300 cubic feet/hour (140 liters/min) of nitrogen from twodownstream gas introduction devices (e.g., manifold 332 in FIG. 3C).This resulted in approximately 140 ppm oxygen concentration in thecontrolled environment of the polymerization section. After exiting theapparatus, the web traveled approximately 3 ft (0.9 m) before entering a30 ft (9.1 m) conventional air floatation drier with all 3 zones set at150° F. (66 C). After drying and before winding, the polymerized anddried coating was post-polymerized using a Fusion UV Systems, Inc.VPS/I600 (Gaithersburg, Md.). The Fusion system was configured with anH-bulb and was operated 100% power at less than 50 ppm oxygen in thecure zone.

The refractive indices of the post-polymerized coatings that wereprepared at different web speeds were measured at 632.8 nm using aMetricon MODEL 2010 prism coupler (Metricon Corporation Inc.,Pennington, N.J.). FIG. 6 shows the effect of web speed on measuredrefractive index for constant coating thickness and a constant UV LEDpower setting of 13 amps. The results show the refractive index remainsessentially constant (n=1.22) over the entire speed range, from 20 to100 ft/min (6.1 to 30.5 m/min). The flow rate varied at each speed,ranging from 24 cc/min coating solution flow rate at 20 ft/min webspeed, to 120 cc/min coating solution flow rate at 100 ft/min web speed.

FIG. 7 shows the refractive index versus the UV LED power range for aconstant web speed of 100 ft/min (30.5 m/min), and a coating solutionflow rate of 120 cc/min. With power levels less than about 2 amp, thefinal coating morphology did not change and the refractive index (n=1.5)was that expected of 906 Hardcoat after sequential coating, drying, andcuring. However with increasing power level, the refractive indexprogressively dropped reaching a minimum value (n=1.21) at a powersetting above 9 amps.

Example 4 Effect of Polymerization Chamber O₂ Concentration onRefractive Index

The process described in Example 3 was repeated, with the followingexceptions: the samples were coated at a speed of 100 ft/min (30.5m/min), a coating solution flow rate of 120 cc/min, and 13 amp UV LEDpower was used. The effect of polymerization section oxygenconcentration for two different nitrogen flow conditions was determined.The first condition, with a nitrogen flow rate of 300 cubic feet/hour(140 liters/min) and a resulting oxygen concentration of 140 ppm,produced a coating having a refractive index of 1.21. The secondcondition, with a nitrogen flow rate of 100 cubic feet/hour (46.7liters/min) and a resulting oxygen concentration of >10,000 ppm,produced a coating having a refractive index of 1.45.

Example 5 Effect of Solvent Content of First Solution

Preparation of Modified Nanoparticle Dispersion:

In a 2 L three neck flask, equipped with a condenser and a thermometer,360 g of Nalco 2327 colloidal silica (40% wt solid) and 300 g of1-methoxy-2-propanol were mixed together under rapid stirring. Afterthat, 22.15 g of Silquest A-174 silane was added, the mixture wasstirred for 10 minutes, and 400 g of additional 1-methoxy-2-propanol wasadded. The mixture was heated 85° C. using a heating mantle for 6 hours.The resulting solution was allowed to cool down to room temperature.Most of the water/l-methoxy-2-propanol solvents (about 700 g) wasremoved using a rotary evaporator under a 60° C. water-bath, resultingin a 44% wt A-174 modified 20 nm silica clear dispersion in1-methoxy-2-propanol.

Preparation of Coating Solutions:

70.1 g of the A-174 modified silica dispersion and 20.5 g of SR 444 weremixed together by stirring, to form a homogenous coating solution of56.6% solids solution in 1-methoxy-2-propanol. Several differentdilutions of the homogenous coating solution were prepared, ranging from10% solids to 50% solids. Each dilution included 2% (by weight) of thephotoinitiator Irgacure 184 and was diluted using a 2:1 mixture ofIPA/Dowanol™ PM. Each of the first coating dilutions was processed usingthe 16×22 array of 395 nm UV LED apparatus of Example 3, under the sameprocess conditions as described in Example 2, at a UV LED power of 13amps, with the exception that the nitrogen flow rate was 75 cubicfeet/hour (35 liters/min). The refractive index of each of thepolymerized coatings was measured as described elsewhere, and theresults plotted in FIG. 8.

Although not wishing to be bound by theory, it appears that at lowpercent solids the insoluble polymer matrix does not have enough curedpolymer for mechanical integrity, so shrinkage and densification occursas the film dries; at high percent solids, there may not be sufficientsolvent to make sufficient nanovoids necessary to result in a low indexcoating.

Example 6 Effect of Coating Thickness and UV Dose

Preparation of Coating Solution:

The modified nanoparticle dispersion of Example 5 (A-174 silicadispersion) was prepared. 70.1 g of the A-174 silica dispersion, 20.5 gof SR 444, 1.1 g of Irgacure 184, and 80.4 g of isopropyl alcohol weremixed together by stirring to form a homogenous 30% solids (by weight)coating solution.

Several different coatings were prepared by changing the flow rate ofthe first coating solution, resulting in wet coating thicknesses of 9.7microns (coating A), 12.9 microns (coating B), and 19.3 microns (coatingC). Each of the coatings were processed using the 8×20 array of 385 nmUV LED apparatus under the same conditions as described in Example 2, ata UV LED power varied in 1-amp increments from 0 to 8 amps. Therefractive index of each of the polymerized coatings was measured asdescribed elsewhere. The results are plotted in FIG. 9, showing therefractive index of each of the coatings as a function of UV lamp power.

Example 7 Effect of Photoinitiator Concentration

The coating solution of Example 6 was prepared, with the exception thatthe Irgacure 184 concentration was varied from 1% to 5% by weight ofsolids, in 1% increments, to form five different coating solutions. Theflow rate of each of the coating solutions was set to provide a wetcoating thicknesses of 16 microns. Each of the coatings were processedusing the 16×22 array of 395 nm UV LED apparatus of Example 3, under thesame conditions as described in Example 2, and, at a UV LED power of 13amps. The refractive index of each of the polymerized coatings wasmeasured at each of the photoinitiator levels, as described elsewhere.The results are plotted in FIG. 10, showing the refractive index of thecoatings as a function of photoinitiator concentration.

Example 8 Effect of UV LED Dose

The coating solution of Example 5 was prepared, with the exception thatthe homogeneous coating solution was diluted using a 2:1 mixture (byweight) of IPA/DOWANOL™ PM to 25% solids by weight (the photoinitiatorremained Irgacure 184 at 2% by weight of solids). The flow rate of eachof the coating solutions was set to provide a wet coating thickness of16 microns. A series of constant speed (CS) samples was processed usingthe 16×22 array of 395 nm UV LED apparatus of Example 3, under the sameconditions as described in Example 2, at a web speed of 5 feet/min, andthe UV LED power was changed from 1 to 13 amps in 1 amp increments. Asecond series of constant dose (CD) samples was processed using the16×22 array of 395 nm UV LED apparatus of Example 3, under the sameconditions as described in Example 2, but as the UV LED power waschanged from 1 to 13 amps in 1 amp increments. For the CD samples, theweb was stopped under the LEDs so that the coating experienced the sameUV dose. The UV dose was measured using a High Energy UV IntegratingRadiometer (Power Puck®, available from EIT Inc., Sterling Va.), and theresults are shown in Table 2, along with the time that the web wasstopped under the LEDs for constant dose.

TABLE 2 Time Stopped Setting UV-A UV-C Visible under LEDs (Amps) (J/cm²)(J/cm²) (J/cm²) (sec) 1 0.0104 0.0005 0.07172 104 2 0.0208 0.001 0.1434452 3 0.0312 0.0015 0.21516 34.6 4 0.0416 0.002 0.28688 26 5 0.052 0.00250.3586 20.8 6 0.0624 0.003 0.43032 17.3 7 0.0728 0.0035 0.50204 14.8 80.0832 0.004 0.5736 13 9 0.0936 0.0045 0.64548 11.5 10 0.104 0.0050.7172 10.4 11 0.1144 0.0055 0.78892 9.5 12 0.1248 0.006 0.86064 8.6 130.1352 0.0065 0.93236 8

The refractive index of each of the polymerized coatings was measured ateach of the UV dose levels, as described elsewhere. The results areplotted in FIG. 11, showing the refractive index of the coatings as afunction of UV LED power for constant dose (CD) and constant speed (CS)samples. Due to inerting of the UV chamber, drying and curing can occursimultaneously in this Example.

Example 9 Nanovoided Coating without Addition of Nanoparticles

A radiation curable resin (SR-444) was diluted using a 2:1 mixture (byweight) of IPA/DOWANOL™ PM to 30% solids by weight, and Irgacure 184 wasadded at 2% by weight of solids. The coatings were processed using theapparatus under the same conditions as described in Example 2, and theUV LED power was varied in 1-amp increments from 0 to 8 amps. Therefractive index of each of the polymerized coatings was measured asdescribed elsewhere. The results are plotted in FIG. 12, showing therefractive index of each of the coatings as a function of UV lamp power.

Unless otherwise indicated, all numbers expressing feature sizes,amounts, and physical properties used in the specification and claimsare to be understood as being modified by the term “about.” Accordingly,unless indicated to the contrary, the numerical parameters set forth inthe foregoing specification and attached claims are approximations thatcan vary depending upon the desired properties sought to be obtained bythose skilled in the art utilizing the teachings disclosed herein.

All references and publications cited herein are expressly incorporatedherein by reference in their entirety into this disclosure, except tothe extent they may directly contradict this disclosure. Althoughspecific embodiments have been illustrated and described herein, it willbe appreciated by those of ordinary skill in the art that a variety ofalternate and/or equivalent implementations can be substituted for thespecific embodiments shown and described without departing from thescope of the present disclosure. This application is intended to coverany adaptations or variations of the specific embodiments discussedherein. Therefore, it is intended that this disclosure be limited onlyby the claims and the equivalents thereof.

What is claimed is:
 1. A process for producing a nanovoided article, comprising: providing a first solution comprising a polymerizable material in a solvent, wherein the first solution further comprises nanoparticles, and wherein a weight ratio of polymerizable material to the nanoparticles in the first solution is greater than about 30:70; at least partially polymerizing the polymerizable material with ultraviolet (UV) radiation produced by at least one light emitting diode (LED) to form a composition comprising an insoluble polymer matrix substantially bicontinuous with a second solution, wherein the insoluble polymer matrix comprises a plurality of nanovoids that are filled with the second solution, and wherein at least some of the nanoparticles are bound to the insoluble polymer matrix during the polymerizing step; and removing a major portion of the solvent from the second solution.
 2. The process of claim 1, wherein the polymerizable material comprises a crosslinkable material, and at least partially polymerizing comprises crosslinking the crosslinkable material.
 3. The process of claim 1, wherein the solvent comprises an organic solvent.
 4. The process of claim 1, wherein the solvent comprises a blend of at least two organic solvents.
 5. The process of claim 1, wherein removing a major portion of the solvent comprises drying in a thermal oven, drying with infrared light sources, vacuum drying, gap drying, or a combination thereof.
 6. The process of claim 1, wherein a weight percent solids of the first solution is greater than about 5% and less than about 90%.
 7. The process of claim 1, wherein a weight percent solids of the first solution is greater than about 10% and less than about 60%.
 8. The process of claim 1, wherein a weight percent solids of the first solution is greater than about 30% and less than about 40%.
 9. The process of claim 1, wherein the nanoparticles comprise surface modified nanoparticles.
 10. The process of claim 9, wherein the surface modified nanoparticles comprise reactive nanoparticles, non-reactive nanoparticles, or combinations thereof.
 11. The process of claim 10, wherein a substantial portion of the reactive nanoparticles form a chemical bond with the insoluble polymer matrix.
 12. The process of claim 10, wherein a substantial portion of the non-reactive nanoparticles form a physical bond with the insoluble polymer matrix.
 13. The process of claim 1, wherein a weight ratio of the polymerizable material to the nanoparticles ranges greater than 30:70 to about 90:10.
 14. The process of claim 1, wherein the first solution further comprises a photoinitiator.
 15. The process of claim 1, wherein the at least one LED comprises a peak wavelength at 365, 385, 395 or 405 nanometers. 